A commonly accepted biological waste water treatment is the "activated sludge" method. It is critical that the activated sludge be in sufficient concentration to effectively purify the waste water. The process is continuous flow; as raw waste water enters the activated sludge plant, it is aerated in an aeration chamber and then introduced into a settling tank. Clarified water is drawn from the settling tank and the settled sludge is partially recycled back into the aeration chamber to maintain the desired concentration of active biomass.
As living organisms synthesize new cells and die, "activated" sludge is created. Surplus sludge is diverted into a sludge handling system, also referred to as a "clarifier". In this sludge handling system, the surplus sludge is first concentrated, and then sent to a sludge digester to further reduce its volume and organic content. The digested sludge is then transported to a landfill site for final disposal.
The primary processing of the waste water through the aeration chamber and the settling tank typically takes between 12 and 18 hours. The secondary processing of the excess sludge typically takes from 24 hours to 7 days depending upon the efficiency of the sludge handling system.
The activated sludge process unavoidably produces an enormous amount of excess sludge. The ongoing production of sludge is considered to be a positive sign that the process is operating properly. If the aeration chamber were to produce reduced quantities of sludge or stop producing sludge altogether that would be considered to be a sign indicating an operational problem. The activated sludge must be present in sufficient concentration for contaminants in the waste water to be effectively removed by the microbial activities. The sludge must remain "activated"; ie. it must have a sufficient proportion of living organisms that utilize the contaminants as food or substrate. A reduction in the quantity of sludge is a sign that the microbial consortia has difficulty to maintain their population. If the reproduction rate were to continue to drop the activated sludge would eventually cease to be "activated". With a healthy activated sludge method, approximately 0.4 to 0.8 pounds of sludge is produced for each pound of BOD removed. Approximately 90 percent of the sludge produced eventually must be handled by the sludge handling system. In excess of 30% of the sludge ultimately ends up being disposed of at landfill sites; although this percentage can vary depending upon the efficiency of the waste water treatment process. The secondary treatment and ultimate landfill disposal of excess sludge has enormous costs associated with it.